“Diet” drinks may actually cause weight gain. Scientists have a few theories why.
Updated: Mar 28, 2019
Diet sodas have long been marketed as weight loss tools. The idea is that they allow people to enjoy the sweetness of regular soda, but without all the calories and weight gain.
Scientists, however, are highly skeptical that diet soft drinks, which contain artificial sweeteners, actually deliver on those marketing claims. And evidence seems is mounting that diet sodas and artificial sweeteners may actually promote weight gain.
"The big controversy in this area is whether artificial sweeteners and diet beverages might be contributing to the obesity epidemic and a parallel diabetes epidemic, which is exactly what they’re supposed to help curb," explained Vasanti Malik, a Harvard researcher who has studied diet soda.
A growing body of evidence suggests diet sodas may not be a benign substitute for regular soda
A new study, published last week in PLOS, tracked users of low-calorie sweeteners in soda for 10 years and compared them with people who don’t use artificial sugar. It found that the low-calorie sweetener users were heavier, and had larger waist circumferences and more abdominal obesity than non-users. "These data suggest that low-calorie sweetener consumption may deleteriously affect visceral fat deposition, a strong risk factor for cardiovascular disease and mortality," the researchers concluded.
Another study from 2008 looked at the relationship between consumption of artificially sweetened beverages and long-term weight gain in 3,682 people. Drinking diet soda was associated with an almost doubled risk of overweight and obesity. "These findings raise the question whether artificial sweeteners use might be fueling — rather than fighting — our escalating obesity epidemic," the authors of that study wrote.
Other research on diabetes has come to similar conclusions. A 2015 study in the British Medical Journal analyzed all the best available research on the association between sugar-sweetened beverages, artificially sweetened drinks, juices, and Type 2 diabetes. The findings were startling: Regular consumption of sugary drinks was associated with diabetes — but so was consumption of diet drinks.
There is some contradictory evidence, which makes interpreting these findings tricky. A shorter-term study, published in 2012, found that people who replaced sugary soft drinks with diet beverages actually lost some weight after six months. (In fact, they fared about as well as people who gave up sugary soft drinks and drank water instead.)
"So there are some studies that are reporting that consuming diet beverages actually contributes to weight gain," said Malik. "Others report consuming diet beverages contribute to an increased risk of diabetes. Others say that's not the case, that these studies are flawed." Malik predicts we'll have firmer answers over the next few years, as more studies are done.
Susan Swithers, a professor at Purdue University who has studied artificial sweeteners, agreed that the evidence is still murky. "Anybody who claims the consequences of diet soda for weight are clear is not understanding what the bulk of the literature actually says," she told me.
Part of that murkiness is due to the fact that people who drink diet beverages are fundamentally different somehow from those who don't. Much of the research on diet versus regular soda drinkers involves observational studies, so scientists gather data on what people are already doing (as opposed to running experiments and assigning one group to diet pop and another to sugary soda). Those who choose to drink diet pop over regular soda might do so, for instance, because they already have weight troubles — which would confound the results.
Why diet soda may promote weight gain
Even so, scientists are increasingly questioning whether artificial sweeteners are as benign as they seem, but they’re not sure why diet soda might be so bad for your health. In the absence of a firm answer, researchers have come up with several hypotheses.
First, there's the idea that the act of drinking diet soda might change people's behaviors. Here's how Stanford's Christopher Gardner put it: "If you have a Diet Coke in the afternoon, and then it is dinnertime and you remember that you had a Diet Coke, you might reward yourself with a bowl of ice cream." Capturing this compensation effect is really tough. But it could explain why studies show that diet soda drinkers gain weight and have related health issues.
Or it may be something about the artificial sweeteners in diet sodas themselves. Some scientists have wondered whether these sweeteners affect our gut flora — the bacteria in our digestive tracts that help with metabolism (and many other critical bodily functions).
Emerging evidence — albeit from studies only done on rodents — suggests the chemicals in artificial sweeteners cause disturbances in the gut microflora, which are associated with metabolic disorders like diabetes and obesity. In a new mice study, published in the journal Applied Physiology, Nutrition, and Metabolism, researchers at Massachusetts General Hospital found that aspartame blocks an enzyme called intestinal alkaline phosphatase that was previously shown to prevent obesity, diabetes, and metabolic syndrome.
There’s also research that suggests fake sweeteners affect the brain in weird ways. Diet drinks seem to affect sugar cravings, for one thing. Experimental studies in humans have found that the taste of sweetness, whether real or artificial, can boost appetite and cause people to eat more.
Other small studies demonstrated that when people are given sucralose, they experience a rush of insulin that doesn't lead to a decrease in blood glucose levels, as if the artificial sweetener is preventing the insulin from being as effective as it ought to be. This is the hallmark of insulin resistance and a harbinger of diabetes.
The underlying idea here is that fake sugars may trigger our body’s responses to real sugar. When we taste something sugary, our body is conditioned to release hormones like insulin, so that when the sugar and calories hit our gut we're prepared to metabolize them. When you introduce artificial sweeteners, the body gets ready for sugar — but the sugar then doesn’t arrive.
"The learned responses get blunted or go away," explained Swithers. "Your body says, 'Wait a minute, the last time I tasted something sweet, I didn’t get anything. This time, I don’t know what’s going to happen, so I’m not going to get ready to metabolize that much energy.'" So fake sugar may screw up the body’s ability to metabolize real sugar, which promote the metabolic problems like diabetes and obesity.
This doesn’t mean real soda is any better
That’s not to say that loading up on beverages sweetened with real sugar is a better idea. The evidence is unequivocal: Sugar-filled beverages have been linked to Type 2 diabetes, stroke, heart disease, tooth decay, obesity, and, ultimately, premature death. They're devoid of nutritional value, and deliver a mega dose of calories without any accompanying satiety.
The evidence on artificially sweetened drinks and their health impact is less clear, but it points to potential trouble. So if you're weaning yourself off regular soda, drinking a bit of diet soda for a short period might be better overall for your health and weight. If you’re not hooked on sugary drinks, stick to water — replacing sugar with fake stuff may not be doing your body any favors, and could actually be harmful.